This invention relates to the use of chemical mechanical planarization (CMP) in the manufacture of semiconductor integrated circuits and more particularly to prediction of performance characteristics of polyurethane pads used for CMP of semiconductor wafers.
During fabrication of integrated circuits, it is often desirable to planarize and/or polish the surface of a semiconductor wafer. One method of performing these tasks is referred to as chemical mechanical planarization (CMP). In general, the CMP process involves rotation or random movement of a wafer on a polishing pad in the presence of a polishing slurry. The polishing pad is typically formed of a polyurethane material.
Downward pressure on the wafer against the pad, rotational speed of the wafer and the pad, slurry content and pad characteristics determine the rate at which material is removed from the surface of the wafer, and the uniformity of the resulting wafer surface.
Determination of how long a wafer should be planarized or polished has proven to be a difficult task. An apparatus and method for in-situ measurement of the thickness of a material to be planarized for CMP end point determination is described in U.S. Pat. No. Re. 34,425 to Schultz.
Methods of controlling the pressure exerted on the wafer against the pad, rotational speed or random movement of the wafer on the pad and slurry composition are well known in the art. Condition and performance characteristics of the polyurethane pad are more difficult to determine. The ability of a pad to planarize the surface of a wafer varies substantially from pad to pad and over the life of an individual pad.
After a wafer has been through the CMP process the pad will be conditioned to prepare it for another wafer. The conditioning process comprises a controlled abrasion of the polishing pad surface for the purpose of returning the pad to a state where it can sustain polishing. The ability of the conditioning process to return the pad to a state where it can efficiently planarize an additional wafer is dependent upon the pad itself and the conditioning parameters. After planarizing several hundred wafers, the pad may no longer be useful for planarizing wafers despite the conditioning process.
The ability to predict performance characteristics of new and used polyurethane pads would be a great benefit to users and manufacturers of such pads.
A measurement of chemical bonding of polymer chains within a polyurethane pad manufactured for chemical mechanical planarization (CMP) of semiconductor wafers is used to predict performance characteristics of the pad, and to adjust process parameters for the subsequent manufacture of additional polyurethane pads.
After manufacturing a lot, one pad or a portion of a pad from the manufacturing lot is soaked in an organic solvent which causes the pad material to swell. It is believed that the relative increase in size is indicative of chemical bonding of polymer chains within the pad. The increase in pad size is indicative of the performance characteristics of the pad Statistical Process Control methods are used to optimize the pad manufacturing process. A manufacturing lot may consist of any number of pads which are deemed to have been manufactured under conditions which tend to cause all pads within the lot to have very similar performance characteristics. Measurements of pad performance predictors allow predicted pad characteristics to be available for each pad. The predicted performance characteristics may be used as a measure of quality of the pad, and may also be provided to pad end users.
Pad characteristic measurements may be taken before any wafers are planarized. Measurements may also be taken after each wafer is planarized or at intervals throughout the life of the pad. Repeated use of the pad impacts the polishing/planarizing ability of the pad. During the CMP process, polyurethane pads are often exposed to high pH (9.0 to 13.0) and high temperature (0 to 90 C.) environments. A correlation between fluorescence characteristics and pad performance has been noted in pads that have been exposed to such conditions. In order to predict future performance of a used pad, in-situ fluorescence measurements of the pad are performed. The fluorescence characteristics of the pad are also believed to be indicative of the chemical bonding of polymer chains within the pad, and are used to predict the effect conditioning will have on the pad. The predicted effect of conditioning is then used to predict performance characteristics of the pad. The measurement of pad fluorescence characteristics also allows for worn or substandard pads to be replaced prior to wafer processing.